476 



AUGUSTA LA MONT ON 



the corner of the mouth and nearer to the median line (text-fig. 7). This is the 

 smallest of the five groups, and it is invariably enclosed in a capsule. It alone can 

 be conveniently dissected out entire and completely separated from the surrounding 

 tissues, when it appears as a neat oval body about 3 or 4 mm. in length (text-fig. 4). 

 Text-fig. 5 represents an abnormal mandibular capsule, containing only two ampullae, 

 obtained in a specimen of Raia circularis. 



Text-fig. 3. — Ventral dissection of ophthalmic and inner buccal ampullae, R. clavata. x-J. 



S.O. Superficial ophthalmic ampullse. I.B. Inner buccal ampullae. 



N.A. Nasal aperture. M. Mouth. 



Of the five groups of ampullae, three — -the hyoid, the ophthalmic, and the outer 

 buccal — have canals opening on both dorsal and ventral surfaces ; while two — the 

 inner buccal and the mandibular — have canals opening on the ventral surface only. 



The canals in all cases are distributed in definite groups, constant enough in 

 relative position, course, and manner of divergence from the ampullae to be recognised 

 in each species and to be known by some distinguishing name. The number of canals 

 in a group is not constant for a species, nor indeed are the numbers necessarily equal 

 in the corresponding groups on the right and left sides of the same individual. 

 Nevertheless, there is an average number, characteristic of each group of canals in 

 a species, from which there are no extreme variations, and which justifies statements 

 regarding the numerical superiority of certain groups relative to others, and as to 



